Alcohol is an organic chemical compound that is comprised of an alkyl group (carbon chain) and a hydroxyl group (oxygen–hydrogen bound molecule). More commonly it is used to refer to a class of beverages containing the chemical ethanol, a psychoactive drug that depresses the central nervous system.
Physical Properties of Ethanol
Ethanol is a clear, colorless liquid with a characteristic refined odor. In dilute concentrations it has a sweet taste while in concentrated solutions a burning taste is often observed. It has a density of 0.789 g cm−3, a molar mass of 46.07 g mol−1, and a molar volume of 59 cm3. It melts at 159 K (−130°C) and boils at 352 K (78°C). It is slightly basic having a pH of 7.33, and is both flammable and volatile . Ethanol is classified as a primary alcohol, meaning that its hydroxyl (−OH) group is attached directly to a carbon atom that has at least two hydrogen atoms attached to it as well. This, combined with its short carbon chain, enhances its miscibility (solubility of one liquid with another to form a homogenous solution) and viscosity (resistance to flow). These properties further enable ethanol to more easily participate in hydrogen bonding with other molecules, being able to accept or donate one hydrogen atom, and also render the ethanol molecule more stable than some other organic compounds of similar molecular weights.
Metabolism of Alcohol
The primary route of entry for alcohol into the human body is through consumption (drinking) although it also can be absorbed through the skin. Once ingested, alcohol is transported down the esophagus into the stomach. There, it acts as an irritant by increasing the levels of hydrochloric acid (a chemical that aids in the process of digestion) that are secreted from the stomach lining. Approximately 20% of the alcohol consumed is absorbed through the stomach where it is picked up by small blood vessels and transported directly into the bloodstream. The remaining alcohol passes into the small intestine where most of the remaining 80% is absorbed through the intestinal walls into the bloodstream. From there it is carried to the liver and then on to the rest of the body. Because alcohol is water-soluble, it mixes easily with the bloodstream and is quickly circulated throughout the entire body, where it is likely to come into contact with every major organ including the heart, lungs, and central nervous system.
While in the liver, much of the alcohol is broken down by enzymes through the process of metabolism. One of the primary pathways (though certainly not the only one) for this process involves the enzyme alcohol dehydrogenase (ADH), which catalyzes the oxidation of ethanol into acetaldehyde (CH3CHO). While ethanol itself is not a carcinogen, acetaldehyde is mutagenic, which means that it is very often toxic and carcinogenic as well. It is this chemical that is largely responsible for many of the physiological consequences alcohol exerts on the body. This compound is eventually broken down further by aldehyde dehydrogenase 2 (ALDH2) in the cell’s mitochondria where it is converted into the acetate: acetic acid (CH3COOH), whose effects include depression of the central nervous system. The acetate is eventually metabolized to form either carbon dioxide (CO2) in the heart, skeletal muscle, and brain cells; or to form acetyl CoA (C23H38N7O17P3S), which plays an important role in the production of energy and the neurotransmitter acetylcholine .
The rate at which alcohol is eliminated from the blood has been found to be on average 0.015% per hour, but as high as 0.06% per hour. Given that most standard drinks contain approximately 0.06% alcohol by volume, this estimate suggests that the body can eliminate up to one alcoholic beverage each hour. However, a number of factors influence the rate of metabolism, including a person’s genetics, gender, ethnicity, types and levels of enzymes present in the metabolic system, amount of food present in the stomach, and other variables as well. Recent research has suggested that the rate at which alcohol is absorbed and metabolized can vary by as much as three to four times across individuals. However, new research methods that allow precise dosing and specially grown cultured cells show promise in improving our understanding related to alcohol metabolism .
Effects of Alcohol on the Human Body
The effects of alcohol on the human body are numerous. Perhaps the area most widely studied with respect to the physiological effects of alcohol concerns its impact on the brain. Research has demonstrated that different regions of the brain vary in their levels of sensitivity to alcohol. Unlike other psychotropic drugs, which tend to target the receptors for specific neurotransmitters, alcohol appears to exhibit an influence over a number of areas and in variety of ways. Receptor sites for the neurotransmitters gamma-aminobutyric acid (GABA, implicated in inhibition of anxiety), N-methyl-D-aspartic acid (NMDA, which may be implicated in the physical dependence of alcohol), and serotonin (implicated in mood regulation) are just a few examples of those that have been shown to be specifically influenced by the presence of alcohol .
It has already been noted that alcohol is quickly and easily mixed with the bloodstream and distributed throughout the body. In fact, this process occurs so quickly that alcohol carried by the circulatory system can reach the brain in less than 1 min after consumption. Because of the way the vascular system is structured in relation to the brain, the blood (and consequently the alcohol) reaches the outer cortical areas first, before making its way inward and downward through the brain structures. This pattern suggests that the first areas of the brain to be affected by alcohol are those responsible for higher order processing (affecting such domains as judgment, decision making, and inhibition), followed by lower order processing centers (affecting coordination, memory, emotion, and sensory processing), and finally reaching the more primitive areas of the brain (affecting levels of consciousness and life functions such as breathing and heart rate).
While the initial impact of alcohol on the brain may be immediate, the consequences of prolonged use have longer-lasting effects. Many of these effects are not seen during the childhood or adolescent years (i.e., liver cirrhosis, hepatitis, etc.) because their onset and course generally require longer periods of development than adolescence affords. Despite this, recent studies have begun to demonstrate the negative impact of moderate to heavy alcohol consumption among adolescents. These findings include lower levels of sex hormones (estrogen in females, testosterone in males), lower levels of growth hormones, increased levels of liver enzymes typically identified as indicators of liver damage, and lower bone mineral density . Further, alcohol use in adolescence is associated with decreased hippocampal volume (associated with memory impairment), abnormalities in the corpus callosum, and overall smaller brains when compared to control groups of adolescents who did use alcohol .
Beyond the problems already discussed, there also exists growing evidence that alcohol use during adolescence may further lead to long-term physiological consequences. One suggestion is that because of the rapid growth and development taking place during adolescence, exposure to alcohol during this critical period may disrupt cognitive development in such a way that it may predict future psychological and physiological concerns . There remains much to be learned with respect to the short- and long-term impact of alcohol consumption among adolescents. Moreover, the preliminary findings already discussed, along with the recently observed trends in adolescent alcohol consumption (see “Drinking,” this volume) underscore the need for continued investigation into this important area of research.
One final area that must be addressed concerns the impact of others’ alcohol consumption on child and adolescent development. Children can experience the effects of others’ alcohol use both directly and indirectly. For example, the social environment plays a significant role in adolescents’ decisions to consume alcohol, particularly with respect to the influence of parents and peers . Further, among the more notable examples of direct consequences to childhood development are the fetal alcohol spectrum disorders, including fetal alcohol syndrome.
Fetal alcohol syndrome is one of the leading causes of birth defects as well as a leading cause of mental retardation. The disorder is believed to develop prenatally as a result of heavy drinking by the mother during pregnancy. Because alcohol is distributed so easily throughout the circulatory system and because the developing fetus receives its blood supply from the mother, the mother’s alcohol consumption can have a direct and negative impact on the fetus’s development. The problems associated with fetal alcohol syndrome are many. Children with this disorder tend to exhibit retarded growth; cognitive, neurological, and motor deficits; learning difficulties, and social and behavioral problems. They may also display a number of permanent physical abnormalities, particularly with respect to craniofacial malformations. These can include widely spaced eyes, small eye openings, and skin folds at the corners of the eyes; a short nose and lower nasal bridge; small head circumference, small midface, and receding chin; thin upper lip and a groove between the nose and upper lip. Often brain development is also impaired, with a number of brain structures including the corpus callosum, cerebellum, and basal ganglia showing decreased volume and size.
In summary, alcohol represents a significant problem for children and adolescents throughout their development. The consequences associated with exposure to alcohol during critical developmental periods accentuate the severity of this problem. While a majority of the available research has focused on the effects alcohol has on the brain and central nervous system, it is clear that the consequences are many and far reaching. Many questions still remain regarding the extent to which the chronic and severe alcohol-related problems observed in adulthood will also be observed in adolescence. However, the relationship between early alcohol use and later alcohol-related problems has already been established. Despite the magnitude of the current problems associated with alcohol and adolescent populations, new technologies and research methodologies suggest our understanding of these important issues will continue to improve as future investigations are undertaken.